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In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Results
  • Discussion
  • Disclosures
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

Summary

A synthesis method to obtain porous platinum-based macrotubes and macrobeams with a square cross section through chemical reduction of insoluble salt-needle templates is presented.

Abstract

The synthesis of high surface area porous noble metal nanomaterials generally relies on time consuming coalescence of pre-formed nanoparticles, followed by rinsing and supercritical drying steps, often resulting in mechanically fragile materials. Here, a method to synthesize nanostructured porous platinum-based macrotubes and macrobeams with a square cross section from insoluble salt needle templates is presented. The combination of oppositely charged platinum, palladium, and copper square planar ions results in the rapid formation of insoluble salt needles. Depending on the stoichiometric ratio of metal ions present in the salt-template and the choice of chemical reducing agent, either macrotubes or macrobeams form with a porous nanostructure comprised of either fused nanoparticles or nanofibrils. Elemental composition of the macrotubes and macrobeams, determined with x-ray diffractometry and x-ray photoelectron spectroscopy, is controlled by the stoichiometric ratio of metal ions present in the salt-template. Macrotubes and macrobeams may be pressed into free standing films, and the electrochemically active surface area is determined with electrochemical impedance spectroscopy and cyclic voltammetry. This synthesis method demonstrates a simple, relatively fast approach to achieve high-surface area platinum-based macrotubes and macrobeams with tunable nanostructure and elemental composition that may be pressed into free-standing films with no required binding materials.

Introduction

Numerous synthesis methods have been developed to obtain high surface area, porous platinum-based materials primarily for catalysis applications including fuel cells1. One strategy to achieve such materials is to synthesize monodisperse nanoparticles in the form of spheres, cubes, wires, and tubes2,3,4,5. To integrate the discrete nanoparticles into a porous structure for a functional device, polymeric binders and carbon additives are often required6,7. This strategy requires extra processing steps, time, and can lead to a decrease in mass specific performance, as well as agglomeration of nanoparticles during extended device use8. Another strategy is to drive the coalescence of synthesized nanoparticles into a metal gel with subsequent supercritical drying9,10,11. While advancements in the sol-gel synthesis approach for noble metals has reduced gelation time from weeks to as fast as hours or minutes, the resulting monoliths tend to be mechanically fragile impeding their practical use in devices12.

Platinum-alloy and multi-metallic 3-dimensional porous nanostructures offer tunability for catalytic specificity, as well as address the high cost and relative scarcity of platinum13,14. While there have been numerous reports of platinum-palladium15,16 and platinum-copper17,18,19 discrete nanostructures, as well as other alloy combinations20, there have been few synthesis strategies to achieve a solution-based technique for 3-dimensional platinum alloy and multi-metallic structures.

Recently we demonstrated the use of high concentration salt solutions and reducing agents to rapidly yield gold, palladium, and platinum metal gels21,22. The high concentration salt solutions and reducing agents were also used in synthesizing biopolymer noble metal composites using gelatin, cellulose, and silk23,24,25,26. Insoluble salts represent the highest concentrations of ions available to be reduced and were used by Xiao and colleagues to demonstrate the synthesis of 2-dimensional metal oxides27,28. Extending on the demonstration of porous noble metal aerogels and composites from high concentration salt solutions, and leveraging the high density of available ions of insoluble salts, we used Magnus’ salts and derivatives as shape templates to synthesize porous noble metal macrotubes and macrobeams29,30,31,32.

Magnus’ salts assemble from the addition of oppositely charged square planar platinum ions [PtCl4]2- and [Pt(NH3)4]2+ 33. In a similar manner, Vauquelin’s salts form from the combination of oppositely charged palladium ions, [PdCl4]2- and [Pd(NH3)4]2+ 34. With precursor salt concentrations of 100 mM, the resulting salt crystals form needles 10s to 100s of micrometers long, with square widths approximately 100 nm to 3 μm. While the salt-templates are charge neutral, varying the Magnus’ salt derivatives stoichiometry between ion species, to include [Cu(NH3)4]2+, allows control over the resulting reduced metal ratios. The combination of ions, and the choice of chemical reducing agent, result in either macrotubes or macrobeams with a square cross section and a porous nanostructure comprised of either fused nanoparticles or nanofibrils. Macrotubes and macrobeams were also pressed into free standing films, and electrochemically active surface area was determined with electrochemical impedance spectroscopy and cyclic voltammetry. The salt-template approach was used to synthesize platinum macrotubes29, platinum-palladium macrobeams31, and in an effort to lower material costs and tune catalytic activity by incorporating copper, copper-platinum macrotubes32. The salt-templating method was also demonstrated for Au-Pd and Au-Pd-Cu binary and ternary metal macrotubes and nanofoams30.

Here, we present a method to synthesize platinum, platinum-palladium, and copper-platinum bi-metallic porous macrotubes and macrobeams from insoluble Magnus’ salt needle templates29,31,32. Control of the ion stoichiometry in the salt needle templates provides control over resulting metal ratios after chemical reduction and can be verified with x-ray diffractometry and x-ray photoelectron spectroscopy. The resulting macrotubes and macrobeams may be assembled and formed into a free-standing film with hand pressure. The resulting films exhibit high electrochemically active surface areas (ECSA) determined by electrochemical impedance spectroscopy and cyclic voltammetry in H2SO4 and KCl electrolyte. This method provides a synthesis route to control platinum-based metal composition, porosity, and nanostructure in a rapid and scalable manner that may be generalizable to a wider range of salt-templates.

Protocol

CAUTION: Consult all relevant chemical safety data sheets (SDS) before use. Use appropriate safety practices when performing chemical reactions, to include the use of a fume hood and personal protective equipment. Rapid hydrogen gas evolution during electrochemical reduction can cause high pressure in reaction tubes causing caps to pop and solutions to spray out. Ensure that reaction tube caps remain open as specified in the protocol. Conduct all electrochemical reductions in a fume hood.

1. Magnus’ salt derivatives template preparation

NOTE: All salt templates should be chemically reduced within a few hours after preparation as prolonged storage results in a degradation of salt structure. This method describes each platinum-based macrotube and macrobeam product. To obtain additional specific product yield, conduct the method with replicate sets of salt template and reducing agent solutions.

  1. Prepare metal salt solutions.
    1. Add 0.4151 g of K2PtCl4 to 10 mL of deionized water to prepare a 0.1 M (100 mM) solution of “Pt2-”.
    2. Add 0.3521 g of Pt(NH3)4Cl2∙H2O to 10 mL of deionized water to prepare a 0.1 M (100 mM) solution of “Pt2+”.
    3. Add 0.2942 g of Na2PdCl4 to 10 mL of deionized water to prepare a 0.1 M (100 mM) solution of “Pd2-”.
    4. Add 0.2458 g of Cu(NH3)4SO4∙H2O to 10 mL of deionized water to prepare a 0.1 M (100 mM) solution of “Cu2+”.
    5. Vigorously shake and vortex platinum and copper salt solutions to aid in the dissolution of the salts until they are fully dissolved.
  2. Prepare platinum salt needle templates.
    1. To prepare Magnus’ salts with a 1:1 Pt2+:Pt2- ratio, pipette 0.5 mL of 100 mM K2PtCl4 into a microfuge tube. Forcefully pipette 0.5 mL of 100 mM Pt(NH3)4Cl2∙H2O into the microfuge tube for a total of 1 mL of salt needle template solution.
      NOTE: The solution will present an opaque light green color. The use of 50 mM K2PtCl4 and Pt(NH3)4Cl2∙H2O will result in longer and wider salt needles for larger platinum macrotubes after chemical reduction29. Forceful pipetting is dispensing the full reagent volume within 1 s to ensure rapid mixing of chemicals within microfuge tubes.
  3. Prepare platinum-palladium salt needle templates.
    NOTE: Salt template platinum-palladium ion ratios are designated as Pt2+:Pd2-:Pt2-. The platinum-only salts prepared in Step 1.2.1. equate to a 1:0:1 ratio.
    1. To prepare the salt ratio 1:1:0, pipette 0.5 mL of 100 mM Pt(NH3)4Cl2∙H2O into a microfuge tube. Forcefully pipette 0.5 mL of 100 mM Na2PdCl4 into the microfuge tube for a total of 1 mL of salt needle template solution.
    2. To prepare the salt ratio 2:1:1, pipette 0.25 mL of 100 mM Na2PdCl4 and 0.25 mL of 100 mM of K2PtCl4 in a microfuge tube. Vortex the microfuge tube for 3-5 s. Then forcefully pipette 0.5 mL of 100 mM Pt(NH3)4Cl2∙H2O into the microfuge tube for a total of 1 mL of salt needle template solution.
    3. To prepare a 3:1:2 salt template solution, pipette 0.167 mL of 100 mM Na2PdCl4 and 0.333 mL of 100 mM of K2PtCl4 into a microfuge tube. Vortex the microfuge tube for 3-5 s. Then forcefully pipette 0.5 mL of 100 mM Pt(NH3)4Cl2∙H2O into the microfuge tube for a total of 1 mL salt needle template solution.
      NOTE: The higher ratio of platinum in the salt templates should yield a greener color, while increasing palladium content results in more orange, pink, and brown color in the solution. Solutions will be opaque in appearance.
  4. Prepare copper-platinum salt needle templates.
    NOTE: Salt template copper-platinum ion ratios are designated as Pt2-:Pt2+:Cu2+. The 1:1:0 ratio equates to the platinum-only salts prepared in Step 1.2.1.
    1. To prepare the salt ratio 1:0:1, pipette 0.5 mL of 100 mM K2PtCl4 into a microfuge tube. Forcefully pipette 0.5 mL of 100 mM Cu(NH3)4SO4∙H2O into the microfuge tube for a total of 1 mL of salt needle template solution.
    2. To prepare the salt ratio 3:1:2, pipette 0.167 mL of 100 mM Pt(NH3)4Cl2∙H2O and 0.333 mL of 100 mM of Cu(NH3)4SO4·H2O into a microfuge tube. Vortex the microfuge tube for 3-5 s. Then forcefully pipette 0.5 mL of 100 mM K2PtCl4 into the microfuge tube for a total of 1 mL of salt needle template solution.
    3. To prepare the salt ratio 2:1:1, pipette 0.25 mL of 100 mM Pt(NH3)4Cl2∙H2O and 0.25 mL of 100 mM of Cu(NH3)4SO4·H2O into a microfuge tube. Vortex the microfuge tube for 3-5 s. Then forcefully pipette 0.5 mL of 100 mM K2PtCl4 into the microfuge tube for a total of 1 mL salt needle template solution.
    4. To prepare the salt ratio 1:1:0, pipette 0.5 mL of 100 mM Pt(NH3)4Cl2∙H2O into a microfuge tube. Forcefully pipette 0.5 mL of 100 mM K2PtCl4 into the microfuge tube for a total of 1 mL salt needle template solution.
      NOTE: The combination of copper and platinum ions forms a purple, cloudy solution that is not as opaque as the solutions of steps 1.2 and 1.3. Leaving solutions of Magnus’ salts for 24 hours or longer will cause the templates to degrade and change to a purple-grey or black color.
  5. Polarized optical microscope (POM) imaging of salt needle templates
    1. Pipette 0.05 mL of salt template solutions prepared in Steps 1.2 – 1.4 onto a glass slide and mount on the stage of a polarized optical microscope. Adjust the focus onto salt needles and rotate cross polarizers until the background is black.
      NOTE: If salt solutions do not present needle-like structures with POM imaging, verify the water quality used for salt solution preparation. Salt needle formation is sensitive to both high and low pH.

2. Salt-template chemical reduction

NOTE: DMAB is toxic. Avoid breathing dust and skin contact by wearing PPE and conduct all associated tasks in a fume hood.

  1. Prepare reducing agent solutions
    1. Add 0.7568 g of sodium borohydride (NaBH4) to 200 mL of deionized water in a 500 mL beaker to prepare a 0.1 M (100 mM) NaBH4 solution. Stir solution with a spatula until the NaBH4 is fully dissolved.
    2. Pour 50 mL of 0.1 M NaBH4 solution into a 50 mL conical tube. Repeat 3x.
    3. Add 1.1768 g of dimethylamine borane (DMAB) to 200 mL of deionized water in a 500 mL beaker to prepare a 0.1 M (100 mM) DMAB solution.
    4. Pour 50 mL of 0.1 M DMAB solution into into a 50 mL conical tube. Repeat 3x.
  2. Adding salts to reducing agent solutions
    1. In a fume hood, pipette the entire 1 mL volume of each of the salt template solutions from Steps 1.2 and 1.3 into each of 4 x 50 mL conical tubes of 0.1 M NaBH4 reducing agent. Allow the chemical reduction to continue for 24 h with the cap off the tube.
    2. In a fume hood, pipette the entire 1 mL volume of each of the salt template solutions from Step 1.4 into each of 4 x 50 mL conical tubes of 0.1 M DMAB reducing agent. Allow the electrochemical reduction to continue for 24 h with the cap off the tube.
      NOTE: Upon the addition of the 1 mL of Magnus’ salts, the reducing agent will turn a cloudy-black color and begin to vigorously form hydrogen gas. Leaving the conical tube caps off prevents the buildup of hydrogen gas pressure and potential explosion and spraying of the solutions. Loose parafilm or foil may be placed over the tubes if dust contamination is a concern.
  3. Rinsing reduced macrotubes and macrobeams
    1. After 24 hours of reduction, slowly decant the supernatant of each of the reduced 50 mL chemical reducing solutions into a waste container and ensure not to pour the samples out of the tubes.
    2. Pour each of the precipitates into new 50 mL conical tubes. The use of a spatula may be required to dislodge sample adhering to the tube sidewalls. Fill each of the new tubes with 50 mL of deionized water and place on a rocker with tube caps secured at a low setting for 24 h.
    3. Remove the tubes from the rocker and place upright in a test tube rack for 15 min to allow the samples to sediment. Slowly pour the supernatant off the top of the tube sample into a waste container. Refill tube with 50 mL of deionized water and place on a rocker with tube caps secured for an additional 24 h.
    4. Remove tubes from the rocker and place upright in a test tube rack for 15 min. Pour the supernatant off the top of the tube into a waste container.
      NOTE: The supernatant will be a clear or grey color and the precipitate will be a black and generally sediment to the bottom of the conical tubes. If pouring the supernatant agitates and resuspends the reduced product, place the tube upright in a rack and wait approximately 15 minutes before pouring again. A small volume of water will remain mixed with the product.

3. Prepare macrotube and macrobeam films

  1. Drying of the samples on glass slides
    1. Pipette as much supernatant as possible out of the 50 mL tubes without removing the reduction product.
    2. Using a spatula, gently transfer the precipitate material to a glass slide. Using a spatula, consolidate the sample into a pile with uniform height of approximately 0.5 mm.
      NOTE: The more water that is removed from the 50 mL tube sample prior to transferring the reduced material to the glass slide, the easier the transfer is. This makes the material behave more like a paste. Sample consolidation and uniform height aids in pressing films after drying.
    3. Place glass slides with the reduced samples in a location that will not be disturbed by air currents. Dry samples for 24 h at ambient temperature.
      NOTE: If more sample is needed for x-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), or other testing, multiple reduced samples from the same salt ratio may be consolidated on the same glass slide for drying.
  2. Pressing of samples and massing the materials
    1. Place a second glass slide on top of a slide with dried mass of reduced samples. With fingers, press down on the glass slide above the material with ample force (approximately 200 kPa) to form a thin film of macrotubes or macrobeams.
      NOTE: Pressing the reduced material between glass slides should result in a free-standing film. Occasionally pressing the dried mass of macrotubes or macrobeams results in multiple film fragments. Films can be trimmed by pressing down with a razor blade.

4. Material and electrochemical characterization

  1. Scanning electron microscopy (SEM): Affix a thin film or lose powder sample with carbon tape on a SEM sample stub. Initially use an accelerating voltage of 15 kV and beam current of 2.7 - 5.4 pA to perform imaging. Zoom out to a large sample area and collect an energy dispersive x-ray (EDS) spectra to quantify elemental composition.
  2. X-ray diffractometry (XRD): Place the macrotube or macrobeam dried sample in a sample holder. Alternatively, place a thin film sample section, as in Step 4.1, on a glass slide. Perform XRD scans for diffraction angles 2Θ from 5° to 90° at 45 kV and 40 mA with Cu Kα radiation (1.54060 Å), a 2Θ step size of 0.0130 °, and 20 s per step.
    NOTE: XRD can be done for either the pressed or un-pressed samples. Powder sample holders typically require a significant volume of materials and the use of pressed thin films is recommended.
  3. X-ray photoelectron microscopy (XPS): Use a monochromated Al Kα source with a 100 μm spot size, 25 W x-ray beam and 45° take-off angle, an operating pressure < 6 x 10-6 Pa. Neutralize surface charging with a low-voltage Ar-ion beam and a barium oxide electron neutralizer. Set analyzer pass energy to 55 eV for high-resolution scans.
  4. Electrochemical characterization
    1. Measure the mass of pressed film samples to normalize electrochemical measurements by milligrams of active materials.
    2. Transfer film samples into an electrochemical vial using either flat tweezers or by gently sliding the film from a glass slide onto the inner sidewall of the vial. Gently pipette 0.5 M H2SO4 or 0.5 M KCl electrolyte over the film samples and let sit for 24 hours.
    3. Use a 3-electrode cell with a Ag/AgCl (3 M NaCl) reference electrode, a 0.5 mm diameter Pt wire auxiliary/counter electrode, and a lacquer coated 0.5 mm diameter platinum working electrode. Place the lacquer coated wire with a 1 mm exposed tip in contact with the top surface of the aerogel at the bottom of the electrochemical vial22.
    4. Perform electrochemical impedance spectroscopy (EIS) from 1 MHz to 1 mHz with a 10 mV sine wave at 0 V (vs. Ag/AgCl).
    5. Perform cyclic voltammetry (CV) using a voltage range of −0.2 to 1.2 V (vs. Ag/AgCl) with scan rates of 0.5, 1, 5, 10, 25, 50, 75, and 100 mVs-1.

Results

The addition of oppositely charged square planar noble metal ions results in near instantaneous formation of high aspect ratio salt crystals. The linear stacking of square planar ions is shown schematically in Figure 1, with the polarized optical microscopy images revealing salt needles that are 10’s to 100’s of micrometers long. A concentration of 100 mM was used for all platinum, palladium, and copper salt solutions. While the salt needle templates are charge neutral in that th...

Discussion

This synthesis method demonstrates a simple, relatively fast approach to achieve high-surface area platinum-based macrotubes and macrobeams with tunable nanostructure and elemental composition that may be pressed into free-standing films with no required binding materials. The use of Magnus’ salt derivatives as high aspect ratio needle shaped templates provides the means to control resulting metal composition through salt-template stoichiometry, and when combined with choice of reducing agent, control over the nano...

Disclosures

The authors have nothing to disclose.

Acknowledgements

This work was funded by a United States Military Academy Faculty Development Research Fund grant. The authors are grateful for the assistance of Dr. Christopher Haines at the U.S Army Combat Capabilities Development Command. The authors would also like to thank Dr. Joshua Maurer for the use of the FIB-SEM at the U.S. Army CCDC-Armaments Center at Watervliet, New York.

Materials

NameCompanyCatalog NumberComments
50 mL Conical TubesCorning Costar Corp.430290
Ag/AgCl Reference ElectrodeBASiMF-2052
Cu(NH3)4SO4Ÿ•H2OSigma-Aldrich10380-29-7
dimethylamine borane (DMAB)Sigma-Aldrich74-94-2
K2PtCl4Sigma-Aldrich10025-99-7
Miccrostop LacquerTober Chemical DivisionNA
Na2PdCl4Sigma-Aldrich13820-40-1
NaBH4Sigma-Aldrich16940-66-2
Polarized Optical MicroscopeAmScopePZ300JC
PotentiostatBiologic-USAVMP-3Electrochemical analysis-EIS, CV
Pt wire electrodeBASiMF-4130
Pt(NH3)4Cl2Ÿ•H2OSigma-Aldrich13933-31-8
Scanning Electron MicroscopeFEIHelios 600EDS performed with this SEM
Shelf RockerThermo ScientificVari-Mix™ Platform Rocker
Snap Cap Microcentrifuge Tubes, 1.7 mLCole ParmerUX-06333-60
X-ray diffractometerPanAlyticalEmpyreanX-ray diffractometry
X-ray photoelectron spectrometerULVAC PHI - Physical ElectronicsVersaProbe III

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